1. Field of the Invention
The present invention relates to a secondary battery which can repeatedly be used. More particularly, this invention relates to a reliable secondary battery capable of preventing short circuits due to dendrite growth, even when the battery is repeatedly charged and discharged.
2. Related Background Art
Since global warming is expected due to the greenhouse effect caused by an increase in CO2 and so forth, the construction of thermal power plants has become problematic. Accordingly, it has been considered feasible to perform so-called load levelling for the purpose of effectively using generators by accumulating electric power at night in secondary batteries at homes to level the load. Another desire has arisen to develop a secondary battery which exhibits a high energy density for use in an electric car that does not exhaust air contamination substances. Further, development of a high performance secondary battery has been needed for use as a power source for portable equipments such as book-type personal computers, word processors, video cameras and portable telephones.
A locking chair type lithium ion battery capable of serving as the foregoing high performance secondary battery and comprising a positive pole activating material comprising lithium ions introduced into an interlayer compound thereof and a negative pole activating material comprising carbon has been developed and partially put into practical use.
However the lithium ion battery has not achieved the high energy density that is the original characteristic of the lithium battery which uses the metal lithium as the negative pole activating material. The reason why a large capacity lithium accumulator of the type that uses lithium metal as the negative pole has not been put into practical use is that the generation of dendrites of lithium (tree branch-like crystals), which are the main cause of short circuiting, cannot be prevented.
The lithium battery, nickel-zinc battery and the air-zinc battery are problematic in that lithium or zinc is, as described above, deposited on the surface of the negative pole at the time of charge. At this time, the current density is locally raised on the negative pole surface depending upon the surface condition, causing lithium or zinc to be selectively deposited in the foregoing place. The deposited metal grows (dendrites) in the form of tree branches upon charging and discharging, while penetrating the separator until it reaches the positive pole, causing a short circuit.
The dendrite reaction mechanism is considered as follows. Since lithium or zinc that deposits at the time of charge has a considerable reactivity, it reacts with electrolytic solution or water or the like in the electrolytic solution, causing an insulating film to be formed which has a large resistance. Therefore, the current density in the foregoing portion is raised at the time of the next charge, resulting in easy dendrite growth. It leads to a short circuit between the negative pole and the positive pole, resulting in that charging cannot be performed.
If the short circuit is extensive, the energy of the battery will be consumed in a very short time, causing the generation of heat. As a result, the solvent of the electrolytic solution can be decomposed, resulting in the generation of gas. When gas is generated, the internal pressure is raised. In the worst, an accidental exposure or fire can be generated. Therefore, there has been a desire for a long life lithium accumulator that does not easily cause internal short circuit even if the charge and discharge cycles are repeated.
Also nickel-zinc batteries and air-zinc batteries generate dendrites of zinc due to repetition of charging and discharging, with the dendrites penetrating the separator. As a result, the zinc negative pole and the positive pole exhibit a short circuit. Therefore, the foregoing conventional technology suffers from an excessively short cycle life.